Retrospective Assessment of Vaccination Coverage
Among School-Aged Children -- Selected U.S. Cities, 1991

Preschool-aged children are at increased risk for
vaccine-preventable diseases; outbreaks of these diseases in this
age group occur predominately among unvaccinated children (1). In
the United States, vaccination rates of individual antigens
required for children at the time they enter school are greater
than 95%; however, vaccination rates for children at their second
birthday are substantially lower (2) despite recommendations by the
Immunization Practices Advisory Committee (ACIP) and the American
Academy of Pediatrics that all children complete a schedule of
vaccination by age 15-18 months.* To retrospectively assess
vaccination levels among school-aged children at their second
birthday, CDC, in collaboration with state and local health
departments, is conducting surveys of vaccination levels among
children entering school who reside in the 60 largest U.S. cities.
This report presents findings from surveys completed in nine
cities ** during 1991.

These surveys use a multistage cluster survey design in which
public and private schools are randomly selected in proportion to
their estimated size (5). At each school, health records for
kindergarten or first-grade students aged 4-7 years are randomly
selected and the dates of vaccinations abstracted. Vaccination
status is assessed for each child at age 3 months (92 days) and at
the second birthday (732 days). Vaccination levels for two
different combinations of vaccine doses are determined: four doses
of diphtheria and tetanus toxoids and pertussis vaccine (DTP),
three doses of oral poliomyelitis vaccine (OPV), and one dose of
measles, mumps, and rubella vaccine (MMR) (4:3:1) or three doses of
DTP, three doses of OPV, and one dose of MMR (3:3:1). In addition,
individual coverage levels are determined for the third and fourth
doses of DTP (DTP3, DTP4), third dose of OPV (OPV3) and one dose of
MMR. *** Coverage rates are calculated both for all vaccines
administered at any age and for only vaccines administered at the
recommended ages and intervals (i.e., strict definition for timing
of valid doses). ****

For the nine cities surveyed, the proportion of children who
were up-to-date with valid doses by their second birthday, based on
the 4:3:1 schedule, ranged from 10% in Houston to 42% in El Paso
(Table 1); however, with a 3:3:1 schedule, vaccination rates were
higher (range: 40% (Houston) to 61% (El Paso)). In addition, when
up-to-date vaccination levels for the 4:3:1 schedule were
determined without application of the strict definitions for timing
of valid doses, the coverage rates increased by 1%-6% for the nine
cities.

When evaluated individually, vaccination levels for specific
antigens were higher than vaccination levels for the complete
vaccination series (Table 1). Vaccination coverage rates by the
second birthday were 11%-47% for DTP4, 53%-77% for OPV3, and
52%-71% for MMR. In the nine cities, 61%-72% of children did not
receive DTP4 at the time they received MMR.

On average, 90% of children had received at least one
vaccination before their first birthday (range: 79% in Miami to 96%
in Cleveland). Of all children surveyed, 83%-98% had had at least
one contact with vaccination services by age 2 years. Most children
began the vaccination series on time: 53%-73% were vaccinated by
age 3 months. Children whose vaccination series was up-to-date by
age 3 months (i.e., had received the first dose of DTP and OPV)
were 3.1 times more likely (range: 2.7-17.0 times) to be up-to-date
with the 4:3:1 combination by their second birthday than those not
up-to-date by age 3 months (Table 2). However, fewer than half
completed their primary series at age 2 years, even among those who
were up-to-date at age 3 months (Table 2).

Editorial Note

Editorial Note: One of the national health objectives for the year
2000 is to vaccinate 90% of children with the primary series by
their second birthday (objective 20.11) (1). However, during the
mid- to late 1980s, vaccination levels in the nine major U.S.
cities reported here were substantially below the stated 90% goal
and lower than levels in other reports (2,6). For example, during
1985, national estimates of vaccination levels for individual
antigens ranged from 77% to 86% for children age 2 years (CDC,
unpublished data, 1985) compared with levels that ranged from 52%
to 84% in this report. However, the 1985 findings were based on a
national sample and did not represent the large urban areas that
constituted the populations sampled for these retrospective
surveys.

Low vaccination rates with the recommended 4:3:1 schedule have
been attributed, in part, to the difficulty of administering the
DTP4 dose on schedule. However, even without DTP4, the rates are
substantially less than the year 2000 objective. Although OPV3 and
DTP4 are both recommended for children at age 15 months, OPV3
coverage substantially exceeded DTP4 coverage primarily because
35%-78% of children receiving OPV3 did so during the first rather
than the second year of life. Furthermore, findings indicated that
many children failed to receive DTP4 at their MMR visit. In 1986,
ACIP recommended that DTP4 and MMR be administered at the same
visit; had these recommendations been in effect and adhered to when
these children were aged 15-18 months, coverage of DTP4 and the
4:3:1 series could have been higher (7). Each contact with
health-care providers represents an opportunity to educate parents
about the recommended vaccination schedule and the importance of
completing the schedule on time.

The findings in this report confirm previous findings
regarding children who had not received the first doses of DTP and
OPV by age 3 months and who, therefore, were at increased risk for
not being up-to-date by their second birthday (8,9). Parents of
children who begin the vaccination series late should be targeted
for intensive education, and greater efforts are needed to track
these children to assure they return for follow-up doses.

To improve vaccination levels by age 2 years among children in
the United States, CDC has begun the Infant Immunization
Initiative. As part of this initiative, each state and local health
department is encouraged to measure current vaccination levels and
develop strategies to improve them. The retrospective survey method
described in this report is easy to implement, can be completed
rapidly and inexpensively, allows different outcome measurements of
vaccination levels at different ages, and provides reliable data
based on school records that are easy to review. Even though these
retrospective surveys cannot detect recent changes in vaccination
levels, when regularly conducted statewide they can be used to
monitor secular trends in vaccination levels.

ACIP. Hepatitis B virus: a comprehensive strategy for
eliminating transmission in the United States through universal
childhood vaccination--recommendations of the Immunization
Practices Advisory Committee (ACIP). MMWR 1991;40(no. RR-13).

CDC. Sampling procedure for conducting immunization
assessment/validation surveys for school and day-care
centers--retrospective surveys using school systems databases and
guidelines for public health immunization clinic audits for
immunization project areas. Atlanta: US Department of Health and
Human Services, Public Health Service, CDC, National Center for
Prevention Services, 1990.

diphtheria and tetanus toxoids and pertussis vaccine; three doses
of oral poliomyelitis vaccine; one dose of measles, mumps, and
rubella vaccine; and a complete series for Haemophilus influenzae
type b --either three or four doses, depending on the type of
vaccine. In late 1991, hepatitis B vaccine was recommended for
universal vaccination of infants (3,4).

** Cleveland; El Paso and Houston, Texas; Miami; New Orleans; New
York City (Bronx); Oakland, California; St. Louis; and Washington,
D.C.

*** Haemophilus influenzae type b vaccination status was not
evaluated because this vaccine is not required for school entry in
all states and data are not available in school records.

**** Strict definition for timing of valid doses are as follows:
the first DTP dose must be given after 42 days (6 weeks) of age
with dose two and three each given after a minimal interval of 28
days. The fourth DTP dose must be given at least 180 days after
dose three. For OPV, the first dose must be given after 42 days (6
weeks) of age, with dose two given a minimum of 42 days after the
first dose. The third dose of OPV must be given a minimum of 42
days after the second dose. Any dose of MMR given on or after the
first birthday was defined as a valid dose. Children with health
records that were not located ( less than 4% of all children
assessed in all schools) were defined as not vaccinated. Only
records that had dates for all vaccinations administered were
assessed as valid.

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